A Cambodian human rights group says one person has been killed and others injured in clashes between police and protesters in the capital Phnom Penh.

An estimated 20,000 protesters are holding three days of demonstrations in response to the country's July election results.

The opposition Cambodia National Rescue party says the poll was rigged in favour of the ruling Cambodian People's Party and wants an independent investigation.

Under the final election results, Hun Sen's Cambodian People's Party won 68 seats, against 55 for the opposition.

Security forces fired smoke grenades, tear gas and water cannon at rock-throwing opposition supporters in an escalation of the political crisis that has gripped the kingdom since the controversial polls.

In one of a series of clashes around the city, anti-riot police fired warning shots and baton charged a group of protesters who were throwing rocks near a bridge some five kilometres from the main protest site, according to a witness.

One demonstrator was killed, according to prominent activist Ou Virak, president of the Cambodian Centre for Human Rights.

I saw with my own eyes he is dead. He was shot in the head

Human rights campaigner Ou Virak

"I saw with my own eyes he is dead. He was shot in the head," he said, adding that while military police had fired "many shots" into the air, the circumstances of the man's death remained unclear.

At least 10 other protesters were injured, Ou Virak said.

"I urge the authorities to ensure that this tragedy is an isolated incident," he said, appealing to both the opposition and authorities to calm the situation, with more rallies planned for Monday.

Military police spokesman Kheng Tito said he was unable to confirm the death, but denied the security forces had fired live ammunition.

"The military police used only batons and shields and police used tear gas. We did not use live rounds," he said.